The German socialite, consumer champion, journalist and spy Inga Haag, who has died aged 91, was an enigma to many who met her for the first time. But for her large circle of friends on both sides of the Atlantic, she was a brave and tenacious woman who had acquired a reputation as the Mata Hari of Marylebone, the area of central London where she was based during her final years. Belatedly, in 2003, the German government awarded her the Cross of the Order of Merit, reassuring sceptics that tales of her wartime exploits were not fantasy, but fact.

Born into a comfortable, upper-middle-class Prussian family towards the end of the first world war, Inga was sent by her liberal banker father to school in England after the rise of Adolf Hitler, because her father was convinced that all German schoolteachers were potential Nazis. Inga was soon fluent in English, later becoming trilingual by adding French, though her use of all three languages was idiosyncratic. She gained entry to the London School of Economics, where she studied under the political theorist Harold Laski, but her family summoned her back to Berlin before the outbreak of the second world war.

Glamorous and intelligent, she was talent-spotted by Admiral Wilhelm Canaris as a useful recruit to German military intelligence. He nicknamed her "the painted doll" and sometimes assigned her to sit in the public rooms of the Adlon hotel in Berlin to listen in to the guests' conversations. Canaris's loyalty to the new regime was ambivalent, to say the least, and Inga soon became a sort of double agent, feeding information about Hitler and his circle to those in the German army and intelligentsia who considered him a jumped-up but dangerous little squirt.

"He had very bad skin," she recalled, many years later, in an interview in the Observer. "He had bad manners and no charm. He had a chicken-skin neck with large pores."

Inga hid her disgust when face-to-face with the Führer, whom she met during the course of her diplomatic duties. But her situation soon became precarious. After the fall of France, Canaris took her to Paris, where she began one of her major wartime undercover operations – supplying false passports to Jews. However, in 1942 she found the perfect cover – as well as a degree of personal happiness – when she accepted a proposal of marriage from a much older, senior army officer, Werner Haag. Like herself, he was a secret dissident, whom she always referred to by the pet-name "Buffalo". The Haags were posted first to Hungary (which Inga hated) and then to Romania (which she loved). Decades later, she proudly displayed on the mantelpiece of her Upper Wimpole Street flat a photograph of Buffalo in full uniform, his Iron Cross to the fore, chatting amiably to the Romanian fascist dictator Ion Antonescu.

In Romania, Inga dutifully played the role of charming hostess to the local hierarchy and Nazi dignitaries while carrying on her clandestine activities, notably keeping contact with a network of well-connected Germans – including the anglophile Adam von Trott zu Solz, to whom she was distantly related – who were plotting to get rid of Hitler. On 20 July 1944, when there was a botched attempt to blow up the Führer, Inga was entertaining two Gestapo officers to lunch in her husband's absence. She knew about the so-called Stauffenberg plot but had to feign surprise and relief at Hitler's survival. Suspecting the witchhunt that would then follow, as soon as the Gestapo officers had left, she gathered all potentially incriminating papers and threw them on the fire, telling her butler that she was destroying letters from a secret lover about whom her husband was becoming suspicious.

Werner was briefly interned by the Americans at the end of the war, but soon released when the true nature of his and Inga's allegiances was verified. They were then posted to Paris, where they worked for Nato, which launched Inga into a new field of engagement, promoting transatlantic relations. The Haags acquired a ground-floor flat in the smart 16th arrondissement and adopted a French boy, Luc Blivet.

Inga was not at all happy about Nato's 1967 move from Paris to Brussels, finding the Belgian capital boring, so she resigned and became a freelance journalist for the International Herald Tribune, among others. Her social life was increasingly rooted in the diplomatic and political worlds of Paris and London. Inga's striking looks and flirtatiousness made her a popular dinner guest. She was thrilled when François Mitterrand played footsie with her under the table, but the future French president was only one of many distinguished admirers who played court to her after Buffalo's early death and the beginning of her long and very merry widowhood.

By the time I met her, more than 20 years ago, when I was summoned by mutual friends to make up the numbers at a weekend luncheon party at her cottage in Lasham, near Alton in Hampshire, she was at the zenith of her social powers, entertaining regularly at her homes in Paris, London and the country, as well as making annual trips to New York, where she operated out of the Harvard Club. In London, one never knew who one would sit next to at table, from Crown Prince Alexander of Yugoslavia, through numerous members of the House of Lords, to leading journalists and various paramours.

I was quickly integrated into what she referred to as her "kindergarten" – a group of younger writers and academics, including Michael Bloch, Giles MacDonogh and, less frequently, Andrew Roberts – who would be summoned to add some youthfulness to the table. The menu was almost always the same, starting with "Bubble" (Spanish cava, served with Ritz crackers), followed by a dry German white wine (and Swedish herrings) and finally claret (to go with the Waitrose main course and cheese). Coffee was always black; Inga was scathing about the supposedly noxious effects of "cow juice".

As the years went by, the guests got fewer and she tended to dominate the conversation, not just with reminiscences but, more often, with details of her latest "mounting the barricades" for some social or environmental cause, such as attacking supermarkets for allegedly forcing up food prices, championing voluntary euthanasia and urging everyone to follow her intended example of having her body disposed of in a black plastic bin-bag.

She appointed me her literary executor: we bonded because we both loathed Christmas, and had become Quakers as a result of our experiences of war. Luc survives her.